The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In currently common drive trains of all-wheel vehicles, only the torque channeled off for the drive of the front axle is controlled by means of a friction coupling. In all-wheel vehicles of the latest generation, however, the torque metered to both axles should be controllable over the total range from 0 to 100 percent. In this manner, the torque metered to the front axle cannot only be regulated in a range from zero up to a proportion fixed by the design and manner of construction, which lies at around 50%, but from 0 to 100%, that is from purely rear wheel drive to purely front wheel drive. All-wheel drive can thus also satisfy all dynamic driving demands and safety demands for fast road driving beyond off-road operation. This also includes the compatibility with electronic systems which act on the brakes of the vehicle. The term “torque vectoring” has become common for this in the technical world.
A drive train of this type is known, for instance, from U.S. Pat. No. 4,709,775. In this, the transfer case adjoining the engine transmission block contains two friction couplings, one in the path to the drive shaft of the front axle and one in the path to the drive shaft of the rear axle. Transfer cases of this type are bulky, expensive and complex assemblies. Above all the substantial requirement of construction space is very problematic subsequent to the transmission, where it is anyway very tight.
A drive train for all-wheel vehicles having two or even four coupling units which meter a regulatable torque to each axle or to each individual wheel, is known from DE 38 14 435 Each coupling unit consists of a controllable liquid friction coupling and of a friction coupling which can be engaged and disengaged, that is a non-controllable friction coupling, for the bridging of the first. The construction effort and the space requirements as well as the regulation problems of this solution are prohibitive. Due to the diversion via the controllable liquid friction coupling, an accurate and fast control is also not even possible.
A drive train is known from U.S. Pat. No. 5,119,298 comprising a transfer case which drives through rigidly to the rear axle and channels off the torque for the front axle by means of the friction coupling. This drive train belongs to the older generation of drive trains which do not permit any variation in the torque distribution between 0 and 100%, but it does show the construction of a transfer case customary in such drive trains.